In a conventionally well known discharge coating apparatus, a suitable voltage pulse is applied between a coating material electrode and a workpiece, and a vibration such as to break the surface of the workpiece at its point is also applied to this electrode synchronously with that pulse. Thus, an electric discharge occurs between the electrode and the workpiece surface, and a portion of the coating material electrode is fused due to the heat produced at this time and is melt-adhered on the workpiece surface, thereby forming a coating layer.
Discharge coating apparatuses are generally constituted as a handy-type tool. The point of the coating material electrode of such apparatus is relatively moved along the workpiece surface, thereby coating a desired portion on the workpiece surface.
As the coating material electrode, various metals and alloy materials are selected in accordance with the purpose of coating, as well as a wear resistant sintered hard alloy, corrosion resisting alloy, fire resisting alloy, etc, which are generally formed into a wire- or rod-shaped, are used. This discharge coating apparatus is provided with a chuck which can co-axially hold these electrodes.
In addition, as an apparatus for causing the coating material electrode to vibrate, a vibrating apparatus comprising an electromagnet which is vibrated by a commercial AC power or a high frequency pulse power supply, springs, etc., or a vibrating apparatus comprising an eccentric cam having an eccentric rotary shaft and a motor to rotate it, and the like are used. The discharge coating apparatus is conventionally constituted in such a manner that these apparatuses allow the chuck, to which the above-mentioned wire- or rod-shaped coating material electrode was attached, to vibrate, and that the point of the electrode comes into contact with the surface of the workpiece and separates therefrom due to the above-mentioned vibrational motion.
On the other hand, a desired voltage pulse is applied when the coating material electrode comes close to or enters into contact with the workpiece surface.
The voltage pulse may have the same frequency as the vibrating frequency of the coating material electrode, but it may be a voltage pulse train having a higher repeating frequency which is intermittent in synchronism with the above vibration. In this case, a plurality of pulse discharges are produced during a certain period of time including one contact period.
Each discharge pulse heats the contact or approaching portion of the workpiece and coating material electrode, causing a portion of the coating material electrode (having particularly small heat capacity) to be melted, so that when the coating material electrode is removed from the surface of the workpiece, melt-adhered coating materials each having a diameter of about 10-30 .mu.m are in the trace of the contact melting point.
In this way the coating material is fused and adheres to the workpiece surface due to the action of gravity and adhesion, whereafter the coating material electrode is removed and the fused metal is resolidified, thereby forming the melt-adhered portion of coating material.
However, the surface of its melt-adhered portion becomes notched because when the coating material electrode is removed from its surface, the fused metal is pulled and stretches as if a thread were pulled due to the surface tention.
It will be possible to carry out desired coatings on the entire or selected portions of the surface of the workpiece by providing a relative machining feed motion to the workpiece surface and/or coating material electrode in a perpendicular direction to the direction of the vibration, and the discharge point is moved on the workpiece surface so as to cover all of the desired portions.
However, in a conventionally known discharge coating apparatus, the coating material electrode is simply and confrontingly disposed on the workpiece surface and this is vibrated perpendicularly against the surface to be coated. Therefore, there are many problems, for example, when the coating material electrode is removed from the surface, the fused coating material is pulled and stretched perpendicularly to the surface as if a thread were pulled and then is finally cut, so that a large projection is produced on the coating surface; on the other hand once such a projecting portion is formed, the contact and melt-adhering of the coating material electrode barely occurs around its projection; as a result of this, the coating material is not uniformly adhered, causing the surface of the coating layer to become notched; moreover, a number of fairly large pin holes and flaw portions are produced; in addition, since the maximum amount that can be coated is small, a thicker coating layer cannot be obtained and the density of the coating layer is insufficient.
In the specification of the U.S. Pat. No. 4,346,281, a plurality of wire-shaped electrodes are annularly held so as to be mutually parallel and the point portions of the wire-shaped electrodes come into substantially parallel contact with the workpiece surface, and a voltage pulse is supplied between the electrodes and the workpiece while rotating the electrodes around the central axis of the assembly of electrodes, thereby forming a coating layer on the workpiece surface.
With this apparatus, the points of the coating material electrodes are rotated along a circular locus which is substantially perpendicular to the workpiece surface, thereby to form a coating layer. However, although this method can provide efficient formation of the coating layer, enough stable mechanical contact is not realized between the coating material electrodes and the workpiece surface, so that a coating layer with sufficiently good quality is not obtained.
Also, in the specification of the U.S. Pat. No. 3,741,426, a rotating electrode is also disclosed, and a voltage pulse train is supplied between the electrode and the workiece while rotating the electrode around the central axis thereof, which is kept parallel to the surface of the workpiece, thereby forming a coating layer on the workpiece surface.
And also, in the specification of Japanese Patent No. 931,227, it is set forth that a rotating electrode is held perpendicular to the surface of the workpiece, and a voltage pulse train is supplied between the electrode and the workpiece while rotating the electrode around the central axis thereof, thereby forming a coating layer on the workpiece surface.
However, in spite of these improvements, a coating layer with sufficiently good quality is not obtained.